As the prices of minerals rise, deep-sea mineral mining becomes an economically-viable alternative to surface mining. There are two primary sources of minerals in the deep sea: “black smokers” and “manganese nodules.”
Black smokers are chimney-like structures, two to five meters in height, which form around breaks in the sea floor. Boiling water that escapes from within the earth through these breaks carries up large amounts of copper, manganese, nickel, gold, cobalt, zinc, and other minerals. As the hot liquid enters the cold deep-ocean water, the entrained metals deposit onto the surrounding ocean floor, forming characteristic columns or chimney-like structures.
Manganese nodules, also called polymetallic nodules, are rock concretions that lie partly or completely buried on the sea floor. The nodules vary in size from microscopic to large coconut-size structures. They vary greatly in abundance; at some locations, the nodules cover more than 70 percent of the ocean bottom. Nodules can be found at any depth, but the greatest concentration is usually found on vast abyssal plains in the deep ocean between 4,000 and 6,000 meters. The total amount of manganese nodules on the sea floor has been estimated at 500 billion tons.
Manganese nodules consist of concentric layers of minerals around a core, which is typically a shell of a microfossil, a phosphatized shark tooth, basalt debris, or fragments from earlier nodules. Nodule growth is extremely slow; approximately one centimeter per several million years. Several processes are involved in the formation of manganese nodules. These processes include: the precipitation of metals from seawater, the remobilization of manganese in the water column, the derivation of metals from hot springs associated with volcanic activity, the decomposition of basaltic debris by seawater, and the precipitation of metal hydroxides through the activity of microorganisms. Several of these processes may operate concurrently or they may follow one another during the formation of a nodule.
The chemical composition of nodules varies according to the minerals present, and the size and characteristics of the core. The nodules of greatest economic interest have the following composition: manganese (27-30%), nickel (1.25-1.5%), copper (1-1.4%) and cobalt (0.2-0.25%). Other constituents include iron (6%), silicon (5%) and aluminum (3%), and lesser amounts of calcium, sodium, magnesium, potassium, titanium and barium.
A major challenge to recovering nodules from the deep ocean is how to economically transport them to the surface. At 6000 meters, pressures are about 6×107 Pascals (about 9000 psi). A minimum of about 60 kilojoules of energy per kilogram of mineral is required for transport to the surface.
Presently, there are only a few ways to transport nodules to the surface. One way is to use an underwater vacuum system. This approach has been demonstrated, but it is quite expensive, both in terms of capital outlay and energy consumption. A second approach uses a vehicle (e.g., a UUV, etc.) that shuttles between the sea bottom and the surface. But this approach is also quite expensive due to capital outlay and energy consumption.
A third approach uses a buoyancy-based recovery system. One such system is disclosed in U.S. Pat. No. 4,010,560, wherein balloons or flexible containers are filled with a gas to lift ore from the sea bed. Using gas to create a buoyant body is difficult and impractical due to the high pressures involved and the high energy costs. A second buoyancy-based system, which is disclosed in U.S. Pat. No. 4,336,662, relies on fixed volume constant-buoyancy bodies (e.g., cork, etc.) that are transported to the ocean bottom using a mass that is discarded upon reaching the ocean bottom. This approach raises environmental concerns (i.e., discarding the material in the ocean) and poses a risk that the discarded material might cover valuable minerals.